使用自定义列表视图适配器时，为什么会出现NPE错误？

时间：2013-02-16 05:07:18

标签： android listadapter

这将是一个冗长的问题，其中包含大量代码，以便那些花时间阅读它的人非常感谢你！

所以基本上我有一个使用Eclipse的应用程序。 MAIN ACTIVITY扩展为FragmentActivity，称为MenuMain.java。此活动在其xml中定义了三个片段。而我遇到问题的是第三个。

我想要做的是当用户点击中间片段上的按钮时，它会将内容添加到自定义列表视图中，这是第三个片段。

这是我在中间片段上显示内容的类。

ImageFragment.java 

package com.thesis.menubook;

import java.util.ArrayList;

import android.app.ProgressDialog;
import android.os.AsyncTask;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentTransaction;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ListView;
import android.widget.RelativeLayout;
import android.widget.TextView;
import android.widget.Toast;

public class ImageFragment extends Fragment {

      ProgressDialog pDialog;
      private  int imageResourceId;
      private ArrayList<String> menu_id =  new ArrayList<String>();
      private ArrayList<String> menu_name =  new ArrayList<String>();
      private ArrayList<String> menu_description =  new ArrayList<String>();
      int page = 0;
      String orderQuantity = "";
      EditText quantity;
      ListView lv;
    public ImageFragment(int imageResource, ArrayList<String> mid, ArrayList<String> mname, ArrayList<String> mdesc, int pg) {
        this.imageResourceId = imageResource;    
        this.menu_id = mid;
        this.menu_name = mname;
        this.menu_description = mdesc;
        this.page = pg;
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

    }

    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);

    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.layout_menu, container, false);

        Button next = (Button)view.findViewById(R.id.next);
        Button back = (Button)view.findViewById(R.id.back);
        Button add = (Button)view.findViewById(R.id.add);

        quantity = (EditText) view.findViewById(R.id.editText1);

        RelativeLayout layout = (RelativeLayout) view.findViewById(R.id.main_layout);
        layout.setBackgroundResource(imageResourceId);

        //Change ID on Layout
        TextView id = (TextView) view.findViewById(R.id.menu_id);
        id.setText(menu_id.get(page));

        //Change name on layout
        TextView name = (TextView) view.findViewById(R.id.menu_name);
        name.setText(menu_name.get(page));

        //change description on layout
        TextView desc = (TextView) view.findViewById(R.id.menu_description);
        desc.setText(menu_description.get(page));

        // set next page button listener
                next.setOnClickListener(new View.OnClickListener(){
                    @Override
                    public void onClick(View v){
                        if(page < menu_id.size()-1)
                        {
                            page++;
                            Toast.makeText(getActivity(), "Next", Toast.LENGTH_LONG).show();
                            loadMenuItem(page);
                        }
                        else
                        {
                            Toast.makeText(getActivity(), "You are at the end of the page", Toast.LENGTH_LONG).show();
                        }
                    }
                });
        // set back page button listener
                back.setOnClickListener(new View.OnClickListener(){
                    @Override
                    public void onClick(View v){
                        if(page != 0)
                        {
                            page--;
                            Toast.makeText(getActivity(), "Next", Toast.LENGTH_LONG).show();
                            loadMenuItem(page);
                        }
                        else
                        {
                            Toast.makeText(getActivity(), "You are at the start of the page", Toast.LENGTH_LONG).show();
                        }
                    }
                });

                //set add to orderlist button listener
                add.setOnClickListener( new View.OnClickListener() {

                    @Override
                    public void onClick(View v) {
                        if(quantity.getText().toString() != "" || quantity.getText().toString() != "0" || quantity.getText().toString() != "00"){
                            lv = (ListView) v.findViewById(R.id.orderListView);
                            new AddToList().execute("");
                        }
                        else
                        {
                            Toast.makeText(getActivity(), "Please put quantity", Toast.LENGTH_LONG).show();
                        }
                    }
                });

        return view;


}
    public void loadMenuItem(int pahina) {

        FragmentManager fm = getFragmentManager();

        if (fm != null) {

            FragmentTransaction ft = fm.beginTransaction();
            ft.replace(R.id.menu_fragment, new ImageFragment(R.drawable.caesar_salad, menu_id, menu_name,menu_description, pahina));
            ft.commit();
        }
    }

    class AddToList extends AsyncTask<String, String, String> {
        ArrayList<DataArray> dataArray = new ArrayList<DataArray>();
        DataArray data = new DataArray();
        /**
         * Before starting background thread Show Progress Dialog
         * */
        @Override
        protected void onPreExecute() {
             super.onPreExecute();
             pDialog = new ProgressDialog(getActivity());
             Log.d("Activity Context", getActivity().toString() + "");
             pDialog.setMessage("Adding to list. Please wait...");
             pDialog.setIndeterminate(false);
             pDialog.setCancelable(false);
             pDialog.show();
        }

        /**
         * Getting product details in background thread
         * */
        protected String doInBackground(String... param) {

            data.setID(menu_id.get(page));
            data.setName(menu_name.get(page));
            data.setQuantity(quantity.getText().toString());
            dataArray.add(data);
            Log.d("Order Data ID: ", data.getID() + "");
            Log.d("Order Data Name: ", data.getName() + "");
            Log.d("Order Data Quantity: ", data.getQuantity() + "");
            return null;
        }

        /**
         * After completing background task Dismiss the progress dialog
         * **/
        @Override
        protected void onPostExecute(String result) {
            pDialog.dismiss();
            lv.setAdapter(new MyCustomAdapter(getActivity(), dataArray));
        }

    }

}

当我在onPostExecute（）上的lv.setAdapter(new MyCustomAdapter(getActivity(), dataArray));行调用我的自定义适配器时，我收到错误

我定义了这样的自定义适配器

MyCustomAdapter.java 

package com.thesis.menubook;

import java.util.ArrayList;

import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseAdapter;
import android.widget.TextView;

public class MyCustomAdapter extends BaseAdapter {
    private static ArrayList<DataArray> orderData;

    private LayoutInflater mInflater;

    public MyCustomAdapter(Context context, ArrayList<DataArray> passed) {
        orderData = passed;
        mInflater = LayoutInflater.from(context);
    }

    public int getCount() {
        return orderData.size();
    }

    public Object getItem(int position) {
        return orderData.get(position);
    }

    public long getItemId(int position) {
        return position;
    }

    public View getView(int position, View convertView, ViewGroup parent) {
        ViewHolder holder;
        if (convertView == null) {
            convertView = mInflater.inflate(R.layout.custom_order_list, null);
            holder = new ViewHolder();
            holder.txtID = (TextView) convertView.findViewById(R.id.id);
            holder.txtName = (TextView) convertView.findViewById(R.id.name);
            holder.txtQuantity = (TextView) convertView.findViewById(R.id.quantity);

            convertView.setTag(holder);
        } else {
            holder = (ViewHolder) convertView.getTag();
        }

        holder.txtID.setText(orderData.get(position).getID());
        holder.txtName.setText(orderData.get(position).getName());
        holder.txtQuantity.setText(orderData.get(position).getQuantity());

        return convertView;
    }

    static class ViewHolder {
        TextView txtID;
        TextView txtName;
        TextView txtQuantity;
    }
}

我的自定义变量DataArray.java是这样的。我没有DataArray的问题，因为它记录了内容，所以我确信NPE错误没有引用我对这个类的使用

package com.thesis.menubook;

公共类DataArray {

    private String menu_ID = "";
    private String menu_name = "";
    private String quantity = "";

    public void setID(String ID) {
     this.menu_ID = ID;
    }

    public String getID() {
     return menu_ID;
    }

    public void setName(String menu_name) {
     this.menu_name = menu_name;
    }

    public String getName() {
     return menu_name;
    }
    public void setQuantity(String quantity) {
     this.quantity = quantity;
    }
    public String getQuantity(){
        return quantity;
    }

}

我认为问题是因为ImageFragment中此行getActivity()中的lv.setAdapter(new MyCustomAdapter(getActivity(), dataArray));。我试过了：

getActivity().getApplicationCOntext()
getActivity().getBaseContext()
getActivity().getParent().getApplicationContext()
getActivity().getParent().getBaseContext()

无效。

那么我该如何解决这个问题呢？

以下是您可能需要分析以帮助我的其他不太重要的类和xml文件。

OrderListFragment.java 

package com.thesis.menubook;

import android.annotation.TargetApi;
import android.os.Build;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

@TargetApi(Build.VERSION_CODES.HONEYCOMB)
public class OrderListFragment extends Fragment{
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.activity_order_list, container, false);
        return view;
    }
}

activity_order_list.xml 

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" >

    <TextView
        android:id="@+id/orderlist"
        android:layout_width="505dp"
        android:layout_height="wrap_content"
        android:paddingLeft="6dip"
        android:paddingTop="6dip"
        android:text="@string/orderlist"
        android:textSize="17sp"
        android:textStyle="bold" />
    <ListView
        android:id="@+id/orderListView"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent" />

</LinearLayout>

custom_order_list.xml 

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical"
    android:paddingBottom="10dip"
    android:paddingLeft="10dip"
    android:paddingTop="10dip" >

    <TextView
        android:id="@+id/id"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textColor="#C20000"
        android:textSize="14sp"
        android:textStyle="bold" />

    <TextView
        android:id="@+id/name"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />

    <TextView
        android:id="@+id/quantity"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />

</LinearLayout>

activity_menu_main.xml 

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id = "@+id/activity_main_large"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="horizontal" xmlns:tools="http://schemas.android.com/tools">

     <fragment
         android:id="@+id/menu_category_fragment"
         android:name="com.thesis.menubook.MenuCategory"
         android:layout_width="0dip"
         android:layout_height="match_parent"
         android:layout_weight="0.14"
         layout="@layout/activity_menu_category" />

     <fragment
         android:id="@+id/menu_fragment"
         android:name="com.thesis.menubook.MenuFragment"
         android:layout_width="0dip"
         android:layout_height="match_parent"
         android:layout_weight="0.71"
         layout="@layout/activity_menu_fragment" />


     <fragment
         android:id="@+id/orderlist_fragment"
         android:name="com.thesis.menubook.OrderListFragment"
         android:layout_width="167dp"
         android:layout_height="match_parent"
         layout="@layout/activity_order_list" />

</LinearLayout>

layout_menu.xml（中间片段内容xml）

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/main_layout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@drawable/caesar_salad"
    android:gravity="top|bottom|center_vertical"
    android:paddingLeft="16dp"
    android:paddingRight="16dp"
    android:paddingTop="16dp"
    android:visibility="visible" >

    <TextView 
        android:id="@+id/menu_name" 
        android:layout_width="wrap_content" 
        android:layout_height="wrap_content" 
        android:text="@string/menu_name" 
        android:textSize="40sp"
        android:background="#2f000000" android:textColor = "@android:color/white"/>

    <TextView 
        android:id="@+id/menu_id" 
        android:layout_width="wrap_content" 
        android:layout_height="wrap_content" 
        android:text="@string/menu_name" 
        android:textSize="40sp"
        android:layout_marginLeft="10dp"
        android:layout_toRightOf="@+id/menu_name"
        android:background="#2f000000" android:textColor = "@android:color/white"
        android:visibility="invisible"/>

    <TableLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/tableLayout1"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:layout_below="@+id/menu_name"
    android:layout_marginTop="20dp" >


    <TableRow
        android:id="@+id/tableRow1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:padding="5dip" >

       <TextView 
        android:id="@+id/menu_description" 
        android:layout_width="wrap_content" 
        android:layout_height="wrap_content" 
        android:background="#2f000000"  
        android:textColor="@android:color/white" 
        android:textSize="20sp" 
        android:textStyle="italic" 
        android:typeface="sans"
        android:text ="@string/menu_description"
        android:layout_margin="10dp">

    </TextView>

         <TextView 
        android:id="@+id/menu_price" 
        android:layout_width="wrap_content" 
        android:layout_height="wrap_content" 
        android:background="#2f000000"  
        android:textColor="@android:color/white" 
        android:textSize="20sp" 
        android:textStyle="italic" 
        android:typeface="sans"
        android:text ="@string/menu_price"
        android:layout_margin="10dp"/>


        </TableRow>

    </TableLayout>


    <TextView 
        android:id="@+id/quantity" 
        android:layout_width="wrap_content" 
        android:layout_height="wrap_content" 
        android:layout_alignLeft="@+id/menu_description" 
        android:layout_alignParentBottom="true" 
        android:layout_marginBottom="20dp" 
        android:background="#2f000000" 
        android:text="@string/quantity" 
        android:textColor="@android:color/white" 
        android:textSize="25sp" 
        android:textStyle="italic" 
        android:typeface="sans"/>


    <EditText 
        android:id="@+id/editText1" 
        android:layout_width="wrap_content" 
        android:layout_height="wrap_content" 
        android:layout_alignBottom="@+id/quantity" 
        android:layout_alignTop="@+id/quantity" 
        android:layout_centerHorizontal="true" 
        android:layout_toRightOf="@+id/quantity" 
        android:layout_marginLeft="5dp" 
        android:background="@android:color/white" 
        android:ems="2" 
        android:maxLength="2"
        android:inputType="number">
        <requestFocus />
    </EditText>

    <Button 
        android:id="@+id/add" 
        android:layout_width="wrap_content" 
        android:layout_height="wrap_content" 
        android:layout_alignBaseline="@+id/editText1" 
        android:layout_toRightOf="@+id/editText1" 
        android:layout_centerHorizontal="true" 
        android:text="@string/add" 
        android:textColor="@android:color/white"/>

    <Button
        android:id="@+id/next"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_above="@+id/tableLayout1"
        android:layout_alignRight="@+id/tableLayout1"
        android:layout_marginRight="42dp"
        android:text="@string/next"
        android:textSize="20sp" />

    <Button
        android:id="@+id/back"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_above="@+id/tableLayout1"
        android:layout_marginRight="26dp"
        android:layout_toLeftOf="@+id/next"
        android:text="@string/back"
        android:textSize="20sp" />

</RelativeLayout>

这是我的LogCat 

02-16 13:25:25.541: D/Order Data ID:(502): 001
02-16 13:25:25.541: D/Order Data Name:(502): Chicken
02-16 13:25:25.541: D/Order Data Quantity:(502): 8
02-16 13:25:25.865: E/AndroidRuntime(502): FATAL EXCEPTION: main
02-16 13:25:25.865: E/AndroidRuntime(502): java.lang.NullPointerException
02-16 13:25:25.865: E/AndroidRuntime(502):  at com.thesis.menubook.ImageFragment$AddToList.onPostExecute(ImageFragment.java:182)
02-16 13:25:25.865: E/AndroidRuntime(502):  at com.thesis.menubook.ImageFragment$AddToList.onPostExecute(ImageFragment.java:1)
02-16 13:25:25.865: E/AndroidRuntime(502):  at android.os.AsyncTask.finish(AsyncTask.java:590)
02-16 13:25:25.865: E/AndroidRuntime(502):  at android.os.AsyncTask.access$600(AsyncTask.java:149)
02-16 13:25:25.865: E/AndroidRuntime(502):  at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:603)
02-16 13:25:25.865: E/AndroidRuntime(502):  at android.os.Handler.dispatchMessage(Handler.java:99)
02-16 13:25:25.865: E/AndroidRuntime(502):  at android.os.Looper.loop(Looper.java:132)
02-16 13:25:25.865: E/AndroidRuntime(502):  at android.app.ActivityThread.main(ActivityThread.java:4025)
02-16 13:25:25.865: E/AndroidRuntime(502):  at java.lang.reflect.Method.invokeNative(Native Method)
02-16 13:25:25.865: E/AndroidRuntime(502):  at java.lang.reflect.Method.invoke(Method.java:491)
02-16 13:25:25.865: E/AndroidRuntime(502):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:841)
02-16 13:25:25.865: E/AndroidRuntime(502):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:599)
02-16 13:25:25.865: E/AndroidRuntime(502):  at dalvik.system.NativeStart.main(Native Method)
02-16 13:25:26.587: D/dalvikvm(502): GC_CONCURRENT freed 126K, 7% free 7635K/8199K, paused 23ms+35ms
02-16 13:25:32.642: I/Process(502): Sending signal. PID: 502 SIG: 9

1 个答案:

答案 0 :(得分：1)

我不完全确定我理解你要完成什么，但在我看来你的列表视图实际上是空的。如果我没有弄错，你只需在onclicklistener的add按钮上初始化它，并使用传递的视图来查找其层次结构中的列表。 onClickListener传递了被单击的实际视图，据我所知，您不能将列表视图作为该按钮视图的子视图，因此findViewById方法返回null。要么初始化列表视图以及按钮，要么确保获得列表的正确父视图，以便在按钮单击时加载它。

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